The invention relates generally to the measurement of particles suspended in a liquid, and particularly to a device for the measurement, over a wide range of concentration, of particles suspended in large bodies of liquids such as in the tanks of waste treatment plants. In such large bodies of liquids, the solids concentrations may vary from a few mg./l. to 25,000 mg./l., and it is often advantageous to measure the concentration of suspended solids at various locations within a given tank, as well as in many tanks within a particular treatment plant. This is usually accomplished by manually drawing samples, with subsequent manual analyzation by a chemist. This is a costly and time consuming procedure and the test results are rarely available in a sufficiently short period of time to be of value for controlling the process.
Such time lag and manpower requirements can be reduced by the use of an automatic suspended solids analyzer at each point of interest. Until now, however, this has been prohibitively expensive because the large number of points and wide range of concentrations has dictated a great number of instruments.
In addition to the initial expense, the presence of a large number of instruments of this nature requires a great amount of maintenance and frequent calibration. Calibration is particularly difficult in a waste treatment plant since the instruments must be removed from their permanent mountings and remounted in a large tank containing a liquid of known concentration, and, to obtain a reliable calibration, this procedure must be performed at several concentrations. An alternate approach is to leave the instrument in its mounting and wait for the concentration of the tanks to gradually change, taking samples at various times. This procedure may take several days since the tanks in a waste treatment plant have long detention times and their contents usually do not change rapidly. When a large number of instruments are present in a given plant, and in need of frequent recalibration, the labor involved in recalibration often approaches the labor of performing the analysis by hand, and the value of the instruments is greatly diminished.
It will be appreciated that the utilization of a portable, wide-range instrument would greatly simplify this measurement problem, quickly providing answers and capable of being readily calibrated. Until now, however, this has not been possible since operation of existing arrangements by means of batteries (1) only renders the problems associated with recalibration portable, (2) the existing designs cannot measure wide ranges of concentrations, and (3) portable operation introduces new problems of its own.
The necessity for frequent recalibration is dependent upon the design of the instrument, the range of its measurements, and how it is employed. In general the causes are:
1. Changes in calibration due to changes in particle size, shape, or color.
2. Changes in instrument indications due to drift. This in turn may be due to:
A. the temperature coefficient of the detectors means. PA1 B. detector light history effects. PA1 C. ambient light interference. PA1 D. temperature coefficient of the emitter means. PA1 E. changes in emitter intensity with source (battery) voltage. PA1 F. obstruction or modification of light paths by slime or debris. PA1 G. amplifier or indicator drift (electronics).
The interaction between these effects is best explained by examining them in accordance with the concentration to be measured.
In low concentrations, the most common measurement method employs the light scattering effect of suspended particles to obtain an indication. Although the exact configuration varies, in general the operation is as follows: A light source illuminates the liquid inside a light tight container and a photocell is positioned so as to receive only light scattered by the particles at some specific angle. The photocell produces only a slight background output in clear liquids and its output increases as more particles are introduced. This method has the following characteristics:
1. Extremely high sensitivity to low concentrations of particles, allowing simple circuitry.
2. Extremely high sensitivity to ambient light, necessitating a fully enclosed chamber to prevent interference.
FIG. 3. Extremely high sensitivity to particle reflectively, thus changing its response with variations in particle size, shape and color.
4. Subject to interference from the absorption and reflection of light by particles adhering to the measurement surfaces.
5. Nonlinear indications, reversing its readings in very high concentrations and thus producing ambiguous indications.
Some of the problems of light scattering instruments can be overcome by employing multiple photocells, or a compensating photocell to measure the amount of light transmitted directly through the liquid. This design modification does not eliminate the requirement for a light tight enclosure. Depending on the configuration of the photocells and electronic signal processing, the use of dual detectors can eliminate the drift associated with temperature changes. It can also reduce the effects of particles adhering to the measurement windows, if and only if, both detectors are positioned at equal angles to the incident light beam and there are no shields or other physical structures in the light beam. It should be noted that this requirement is difficult to obtain without resorting to a design that is subject to ambient light interface and this problem is aggrevated by the fact that two or more cells must be protected. Even so, light scattering techniques are still subject to the basic problem of varying indications as particle reflectivity changes.
In concentrations higher than several hundred milligrams per liter, scattering-type instruments are completely inappropriate due to their nonlinearity, and light transmission instruments are employed. These devices operate by transmission of a beam of light directly through a liquid and detection of the amount of light transmitted (or absorbed) by the particles in the liquid. In clear liquids, the transmission is 100%, decreasing as the solids content increases. Light transmission instruments have the following characteristics:
1. Reverse reading; i.e., full scale at zero concentration.
2. Logarithmetic response in many liquids.
3. Insensitivity to reasonably large variations in particle size, shape, and color (reflectivity).
4. Subject to offsets as slime or debris accumulates on the measurement surfaces. These offsets affect all readings equally; for example, a given amount of debris may produce a 50ppm error and this 50 ppm error will be constant at any reading.
5. Subject to ambient light interference, but to a lesser degree than scattering instruments.
6. Upper limit of measurement determined by detector sensitivity and light source strength.
Both light transmission and light scattering instruments are subject to errors due to changes in either the light source, the detector, or debris on the windows through which the light beams passes.
While nearly all of the characteristics of a transmission or scattering-type instrument can be tailored to the measurement parameters of a particular liquid or environment, previous workers in the field have been unable to produce an efficient portable, wide range instrument.
Available suspended solids analyzers, having any degree of precision, have as a fundamental requirement of their design one of the following objections:
1. A highly focused optical system with multiple lense or baffles which are fragile and cannot withstand abuse. (When shock mounted, such structures are generally heavy and bulky.)
2. Special sample handling, such as pouring into a test tube or pumping through a pipe.
3. Intricate structures that allow rapid accumulation of debris.
4. Complicated mechanisms or methods of calibration.
5. Highly inefficient use of light, resulting in high power consumption and bulky power supplies.